What Is That Thing?

That thing is a CAPTCHA, a "Completely Automated Public Turing test to tell
Computers and Humans Apart". Which is to say, it's a way for software to know
whether it's a human or a machine that's talking to it. You can read more about
CAPTCHAs in general at wikipedia.

In order to satisfy my CAPTCHA, you need only to identify the one string of characters
within a randomly selected sentence that is not an American English word, and type it
into the text input field provided. Admittedly, it's an inconvenience, but it's one
that has become necessary due to the fact that spammers have developed software that
knows how to exploit web-to-mail gateways. I/we cannot employ our other spam rejection
and/or filtering techniques to our web-to-mail gateway because that would leave those
(very rare few) people whose servers are erroneously (or at least inconveniently)
blocked unable to contact us. Meanwhile, we cannot leave our web-to-mail gateway wide
open to the flood of spam that would overwhelm us, so I've chosen an accessible CAPTCHA,
developed wholly in-house, as a, well, least unhappy medium for all concerned.